Table 1.

Original variables (wavenumbers) from PCA with more impact on the first three principal components and the vibrational modes associated with

Principal component	Order of relevancea (#)	Wavenumber (cm−1)		Associated to
		Band (from–to)	Maximum at
PC1	1	2870–2990	2920	C–H and C–H2 symmetric and asymmetric stretching
	2	2800–2870	2855	C–H and C–H2 symmetric and asymmetric stretching
	3	7915–8890	8330	C–H second overtone of –CH2, –CH3 and –CH=CH–
	4	6625–7255	6875	O–H first overtone
	5	4245–4495	4325	C–H stretching and C–H deformation of –CH2 and –CH3
PC2	1	1700–1770	1745	Stretching of the ester carbonyl groups
	2	2870–2990	2920	Described above
	3	2800–2870	2855	Described above
	4	4245–4495	4325	Described above
	5	4970–5390	5170	O–H stretching and deformation
PC3	1	4245–4495	4325	Described above
	2	1490–1570	1540	C–N stretching, C–O stretching and C–C stretching (Amide II)
	3	4970–5390	5170	Described above
	4	1570–1705	1640	C=O stretching and N–H bending (Amide I)
	5	5380–5990	5705	C–H stretching first overtone

^aThe order of relevance of each principal component to each original variable (frequencies) was found after calculating the modulus of each loading value for each component and sorting that values from the highest to the lowest